Satellite navigation has become a prerequisite for a wide range of electronic positioning devices that include products intended for vehicular and portable applications. As a result, deep urban and indoor areas are becoming increasingly important for emerging satellite navigation designs. Attenuation, shadowing, and multipath fading effects in urban canyons and indoor areas frequently degrade the received satellite navigation signals. Signal obstructions in these environments often lead to limited service availability.
A navigation system in use today for determining a position of a mobile radio receiver may utilize the satellite-based GPS system, and in the near future, the European Galileo system. The terms “GPS” and “Galileo” are used interchangeably herein. Both systems work similarly, each employing about 24 to 31 orbiting satellites, each satellite with accurately known position and time that transmit a signal with a time stamp that indicates when the signal was sent from the satellite. In order to be able to compute the position of the mobile receiver, satellite clocks in each system are accurately synchronized to a common time reference. The mobile receiver calculates its position by the following (simplified) steps: First the time stamp of at least four satellites is extracted from received signals by the mobile receiver, and the time of arrival of the time-stamped message is recorded. The distance from each satellite to the mobile receiver is calculated by comparing the respective time stamp with its arrival time, using the accurately known propagation velocity of the radio signal. The position of each satellite can be accurately determined at any instant in time by the mobile receiver using satellite ephemeris data, which is regularly updated and transmitted by the satellites. Thus, the distance to at least three of the satellites can be calculated by the mobile receiver position using triangulation. However, the clock in the mobile receiver may not be accurately synchronized with the synchronized clocks in the satellites. A time-stamped signal received from a fourth satellite by the mobile receiver is generally used to compensate clock uncertainty in the mobile receiver. To improve accuracy even further in determining the location of the mobile receiver, perturbation effects operative on the received signal such as atmospheric effects, earth rotation, relativity, etc., are typically included in the computation of the mobile receiver location.
GPS satellites transmit on carrier frequencies of 1.57542 GHz (for the GPS L1 signal) and 1.2276 GHz (for the GPS L2 signal). The GPS carrier is modulated with a spread-spectrum technique employing a pseudorandom code with a bit rate of about 1 Mchip/s (1.023 Mchip/s) for the coarse acquisition (“C/A”) GPS code, and 10.23 Mchip/s for the precise (P) GPS code. Thus, roughly 100-2000 carrier cycles, depending on the GPS signal, comprise one spread-spectrum chip. Since the speed of light (in a vacuum) is about 299,792,458 m/s, the “length” of one chip at 1 Mchip/s is about 300 m, and at 10 Mchip/s, about 30 m. This means that in order to obtain good accuracy in the position computation, the arrival time must be determined by the mobile receiver within a reasonably small fraction of a chip. Existing products are able to determine position with accuracy well better than 10 m when there is sufficiently high signal-to-noise ratio, confirming that it is possible to determine the time of arrival of the received signal with high accuracy.
Practical satellite navigation applications thus require Galileo/GPS receivers with high positioning accuracy at low signal-to-noise ratio. This is particularly the case for applications in deep urban and indoor environments where envelopes of buildings and vehicles attenuate the signals transmitted by the satellites, which are already quite weak at about −158.5 dBW when unobstructively received on the earth's surface. The result is a degrading of positioning accuracy in urban and indoor environments in view of user expectations or system requirements.
Enhanced reception sensitivity is a key success factor for satellite navigation in the mass consumer market. Galileo/GPS receivers with higher reception sensitivity would enable more widespread utilization of satellite navigation.
Thus, the design of an improved Galileo/GPS receiver that provides improved signal detection and, correspondingly, improved positioning availability and accuracy, particularly in an environment of low received signal-to-noise ratio, would address an unanswered application need.